NASA commentator Rob Navias said the new module passed an initial pressure test. "We have a pump that is alive and well," he reported on NASA TV.

The only other time NASA conducted a spacewalk on Christmas Eve was during a Hubble repair operation in 1999. The timing lent a holiday spirit to Tuesday's proceedings. "It's like Christmas morning, opening up a little present here," Mastracchio joked as he checked his tools.

Two and a half hours into the job, Hopkins stood at the end of the robotic arm and steadied the 780-pound (355-kilogram) pump module as it was swung into position for installation. "Mike Hopkins taking a special sleigh ride on this Christmas Eve," Navias observed.

The spacewalk didn't always go as smoothly as Santa's rounds, however: After the astronauts slid the boxy apparatus into place and secured it with bolts, they had some trouble switching the cooling system's fluid lines. They had to tap and pry at one of the interim fluid-line connections to free it up, and in the process they set off a mini-blizzard of toxic ammonia "snowflakes."

The snowflakes dissipated, and all the proper connections were eventually made. But the astronauts reported that some of the frozen ammonia got onto their spacesuits. As a result, they had to take some extra time to let the chemicals "bake out" from their suits before ending the spacewalk.

Essential roleTuesday's spacewalk followed up on Saturday's operation to remove the faulty pump module. A valve inside that module failed on Dec. 11, forcing one of the station's two ammonia coolant loops to go offline.

The cooling system plays an essential role in keeping the onboard electronics from overheating. When the first loop failed, NASA had to shut down non-essential systems and switch other systems over to the second loop, reducing the station's safety margin in the process. If the other loop had failed, that could have forced the six-man crew to abandon the station.

A similar pump module switch-out required three difficult spacewalks in 2010, but only two were needed this time around, in part because of the lessons learned three years earlier. The astronauts who were involved in those 2010 spacewalks, Doug Wheelock and Tracy Caldwell Dyson, were on hand at Mission Control in Houston to lend advice.

When the repairs were made, Wheelock told the crew, "It's the best Christmas ever."

"Merry Christmas to everybody," Hopkins replied. "It took a couple of licks to get 'er done, but we got it."

Worries about waterSafety concerns added some extra twists to the past week's repair operation: In July, Italian astronaut Luca Parmitano almost drowned when water from his spacesuit cooling system backed up into his helmet. The crew overhauled the spacesuits, and NASA pronounced them "clean" — but as a precaution, the helmets were equipped with absorbent pads and emergency snorkels.

Neither Mastracchio and Hopkins reported any helmet problems on Saturday or on Tuesday.

After Saturday's 5.5-hour spacewalk, Mastracchio mistakenly flipped a switch on his suit that may have sent water to a different cooling device known as a sublimator. In a worst-case scenario, that could have caused the device to freeze up during the next spacewalk.

As a result, Mastracchio's suit was set aside to dry out, Hopkins' suit was resized to fit Mastracchio, and a backup suit was fitted for Hopkins' use. The switch required an extra day of preparation — which is why the follow-up spacewalk took place on Tuesday instead of Monday, as originally scheduled.

Getting back to normalIf further tests confirm that Tuesday's repairs were successful, operations could return to normal over the next few days. NASA also could go ahead with a cargo resupply mission that had to be postponed due to the cooling system problem. The launch of Orbital Sciences' Cygnus cargo capsule is now scheduled for Jan. 7.

The station is currently at its maximum capacity of six live-aboard spacefliers. In addition to Mastracchio, Hopkins and Wakata, three Russians round out the crew: Oleg Kotov, Sergey Ryazanskiy and Mikhail Tyurin.

Kotov and Ryazanskiy are planning a spacewalk of their own on Friday, to install new equipment on the station's Russian segment.